Sensors are devices that respond to a stimulus and produce a signal indicative of the stimulus' magnitude or other characteristic related to the stimulus. The stimulus may be any physical quantity or parameter which can affect a sensor and is usually a measurable parameter or effect. An array of sensors is a collection of individual sensors that are positioned at discrete locations and are related to one another in at least some aspects.
Sensor arrays are used in applications such as imaging, and generally involve a plurality of individual sensors placed in relation to one another such that an effectively larger sensor is formed by the array of sensors. That is, when placing sensors at a plurality of discrete locations over a region of interest it is possible to make some determination or estimate of the stimulus over the entire region of interest. Extrapolation or interpolation can provide an estimate of the magnitude of the stimulus at a spot which does not itself contain a discrete sensor. Furthermore, aggregate measures of the stimulus over the entire region of interest or smaller regions within the region of interest may be obtained by averaging or other operations performed on signals derived from individual sensors.
Applications in which such sensor arrays are useful include touch pads and distributed sensors that provide an indication of the location and magnitude of a force or a pressure applied to a region of interest.
One type of sensor array is a capacitive sensor array. This array employs a number of discrete capacitors distributed over a region of the array which may be arranged in a pattern forming a grid. A grid of sensors may comprise a plurality of capacitive sensors which may be individually addressable or addressable in groups or in their entirety. Addressing specific sensors may be accomplished using multiplexers coupled to the sensor array according to data or select signals on multiplexer select lines to determine the individual sensors to be driven or sampled. By driving a sensor it is meant the process of generally exciting the sensor or energizing the sensor so as to produce a measurement of the stimulus at the sensor. By sampling a sensor it is meant receiving an output signal from the sensor to read or detect the sensor response to the stimulus. Thus, it is possible to selectively measure a signal from a given capacitive sensor element located at a particular column and row of the capacitive array. Multiplexers may be used to determine the particular row and column from which a measurement is desired.
Capacitive array sensors have been constructed of rows and columns of conductive strips separated by a dielectric material. FIG. 1 illustrates a capacitive array 100 having conductive strips arranged along rows 102 and columns 104. The rows 102 and columns 104 of the capacitive array 100 may be separated by a flexible deformable material such as a silicone gel. The silicone gel (not shown) will deform in response to pressure applied to a surface of the capacitive array 100. The deformation of the silicone gel or other flexible substance can cause the rows 102 and columns 104 of the capacitive array 100 to become nearer or more distant to one another. Gap distance (d) is a factor which determines the capacitance of the capacitors 200 formed by ah intersection of the rows 102 and columns 104 of the capacitive array 100. If the rows 102 and columns 104 of the capacitive array 100 are coupled to electrical connections and to an external circuit, the capacitance of each of the capacitors 200 formed by the intersection of the rows 102 and columns 104 can be measured individually.
A sensor array can be driven and sampled, one sensor at a time or in groups, or in its entirety. By scanning the capacitive array 100 to obtain a signal or measurement from each of its individual elements 200, it is possible to form a real-time picture of the pressure applied to the capacitive array 100.
FIG. 2 illustrates a single capacitive array element 200. The element 200 is formed by an intersection of a row 102 and a column 104 of the capacitive array 100. The figure illustrates a distance or gap (d) that separates the row 102 and column 104 conductive strips. The capacitance of the capacitor 200 is generally proportional to the area formed by the intersection of the row 102 and column 104 divided by the distance d. Hence, changes in the distance d result in changes in the value of the capacitor 200. The relationship between the capacitance of the capacitor 200 and the stimulus, e.g., applied pressure, may be nonlinear for a variety of reasons. These reasons include the deformation response of the flexible deformable material, e.g., the silicone gel, as well as other physical and electrical responses of the variable gap capacitance element 200.
For large arrays, technical challenges arise in making fast measurements or scans of the entire sensor array. For example, a sampling circuit such as a multiplexer that samples a selected row and column on which to perform a measurement would have to cycle through all rows and all columns (all elements of the array) at a rate sufficient to provide the measurements as required by the specific application.
Nonlinear responses in the signals derived from the individual capacitors and the stimulus, e.g., applied pressure, complicate the design of an overall sensor circuit. Furthermore, the measured signal is typically small compared to the driving signal which drives the capacitive array. This results in a poor signal-to-noise ratio when attempting to derive a useful modulation signal reflecting the quantity being measured. This is because noise becomes amplified as well as the signal being measured when using simple signal amplification.
Traditional sensor circuits employ filters and switches that slow acquisition times by causing transients which need to decay between acquiring measurements from the various elements of an array. For example, in scanning a sensor array, a switch switches between the individual sensors of a traditional array, causing a transient signal to occur. Not only do transients slow the acquisition of a complete sensor array scan, but they can affect the quality of a measurement of a stimulus by introducing noise into sensed signals.
Furthermore, conventional sensor arrays contain considerable parasitic capacitances between sensor elements and other parts of the circuit, such as ground. These parasitic capacitances can contaminate sensed signals with noise and extraneous signal components and can require extra filtering circuitry and processing time to compensate for the parasitic capacitance.